1. Field of the Invention
The invention relates to a process for preparing 2,2′-di(alkylenedioxythiophene)s (also referred to in the literature as 2,2′-bis(3,4-alkylenedioxythiophene)s or 3,4-(alkylenedioxy)-2,2′-bithiophenes), to novel compounds of this class and also to their use as important precursors or for preparing important precursors for π-conjugated polymers.
2. Brief Description of the Prior Art
The class of the π-conjugated polymers, which are also referred to as conductive polymers or synthetic metals, has been the subject matter of numerous publications. Owing to the considerable delocalization of the π-electrons along the main chain, these polymers exhibit interesting (non-linear) optical properties, and after oxidation or reduction, they become good electrical conductors. This allows these polymers to be used in various practical fields of use, for example in data storage, optical signal processing, the suppression of electromagnetic interference (EMI), solar energy conversion, rechargeable batteries, light-emitting diodes (LEDs), field effect transistors, circuit boards, sensors, capacitors and antistatic materials.
Examples of existing π-conjugated polymers are polypyrroles, polythiophenes, polyanilines, polyacetylenes, polyphenylenes and poly(p-phenylene-vinylenes). A particularly important and industrially utilized polythiophene is poly(3,4-ethylenedioxythiophene) which has very high conductivity in its cationic form. Of note is a series of polymers which contain the 3,4-ethylenedioxythiophene building block, which are suitable as materials for light-emitting diodes, owing to their electroluminescence,.
Of particular interest here are 2,2′-di(3,4-ethylenedioxythiophene) building blocks which are useful for the adjustment of the emission wavelengths and of the luminescence intensity. (See, for example, A. Donat-Bouillud, I. Lévesque, Y. Tao, M. D'lorio, S. Beaupré, P. Blondin, M. Ranger, J. Bouchard and M. Leclerc, Chem. Mater. 2000, 12, p. 1931–1936).
A problem associated with this use is that the 2,2′-di(3,4-ethylene-dioxythiophene)s have hitherto been obtainable only via complicated organometallic syntheses which, depending on the reaction conditions, can lead to low yields and/or to only moderate purities.
The 2,2′-di(3,4-ethylenedioxythiophene)s have hitherto generally been prepared in the literature by the following process (Ullmann coupling):3,4-Ethylenedioxythiophene is lithiated under protective gas using n-butyllithium at −78° C. in absolute tetrahydrofuran and subsequently coupled with copper(II) chloride oxidatively to give 2,2′-di(3,4-ethylenedioxythiophene).
Pertinent studies cited hereinbelow illustrate the disadvantages of this synthetic route, in particular with regard to reaction conditions, yields and purities: G. A. Sotzing, J. R. Reynolds and P. J. Steel, Adv. Mater. 1997, 9(10), p. 795–798 shows that only an impure product was obtained (m.p. 183–185° C.). A. Donat-Bouillud,. I. Lévesque, Y. Tao, M. D'lorio S. Beaupré, P. Blondin, M. Ranger, J. Bouchard and M. Leclerc, Chem. Mater. 2000, 12, p. 1931–1936 describe the preparation of a purer product by the same process, (m.p. 203° C.), with yield of only 27.7% of theory, which is rather low. Extension of the reaction time for the Ullmann coupling as reported by A. K. Mohanakrishnan, A. Hucke, M. A. Lyon, M. V. Lakshmikantham and M. P. Cava (Tetrahedron 55 (1999), p. 11745–11754) provided a yield of 84% of a relatively pure product (m.p. 203–204° C.). However, the reaction time had to be increased by a factor of six.
Also, the following significant disadvantages of the processes for preparing 2,2′-di(3,4-ethylenedioxythiophene) are described in all these publications: 1 ) For the lithiation, it is necessary to work at low temperatures (−78° C.) with the aid of an external cold mixture, for example of acetone/dry ice. 2) n-Butyllithium as an organometallic reagent, is expensive and is air- and moisture-sensitive when handled, and also, especially in the event of ingress of moisture, readily flammable. 3) Owing to the sensitivity of the butyllithium and of the lithiated thiophene formed as an intermediate, the entire reaction has to be carried out under protective gas (nitrogen or argon).
The variant of this process provided by S. S. Zhu and T. M. Swager in J. Am. Chem. Soc. 1997, 119, p. 12568–12577 (lithiation at −10° C. using tetramethylethylenediamine and oxidative coupling by means of iron(III) acetylacetonate under reflux in THF). This process results in no simplifications, since cold mixtures and protective gas technology are likewise required: The actual yield is 50% of theory (0.99 g of the maximum possible 1.97 g); the 99% reported results from a printing error.
A further significant disadvantage of the cited organometallic procedures is their limited applicability. 2,2′-di(3,4-ethylenedioxythiophene)s substituted on the ethylene bridge have, therefore, hitherto not been described. The substituents can be, for example, hydroxyalkyl groups, carbonyl or heterocarbonyl groups, double bonds, etc. They would not be accessible by a selective route by the method of lithiation with subsequent Ullmann reaction, since, in the case of numerous conceivable substituents on the ethylene bridge, lithiation of these substituents as a side or main reaction would occur. 2,2′-di(3,4-alkylenedioxythiophene)s having alkylene bridges other than the ethylene bridge have also hitherto not been described in the literature.
There is therefore a need for a novel method for preparing existing and also novel 2,2′-di(3,4-alkylenedioxythiophene)s which                dispenses with the use of low temperatures, i.e. cold mixtures or the like for external cooling,        allows implementation under air without protective gas technology        provides broad applicability, for example for the preparation of 2,2′-di(3,4-ethylenedioxythiophene)s functionally substituted on the ethylene bridge or other optionally substituted 2,2′-di(3,4-alkylenedioxythiophene)s.        